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Laser microstructuring for fabricating superhydrophobic polymeric surfaces
Applied Surface Science 257 (2011) 3281–3284
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laser microstructuring for fabricating superhydrophobic polymeric surfaces M.R. Cardoso, V. Tribuzi, D.T. Balogh, L. Misoguti, C.R. Mendonc¸a ∗ Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, CEP 13560 970 São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Received 23 August 2010 Received in revised form 30 October 2010 Accepted 31 October 2010 Available online 5 November 2010 Keywords: Laser micromachining Polymers Superhydrophobic surfaces
a b s t r a c t In this paper we show the fabrication of hydrophobic polymeric surfaces through laser microstructuring. By using 70-ps pulses from a Q-switched and mode-locked Nd:YAG laser at 532 nm, we were able to produce grooves with different width and separation, resulting in square-shaped pillar patterns. We investigate the dependence of the morphology on the surface static contact angle for water, showing that it is in agreement with the Cassie–Baxter model. We demonstrate the fabrication of a superhydrophobic polymeric surface, presenting a water contact angle of 157◦ . The surface structuring method presented here seems to be an interesting option to control the wetting properties of polymeric surfaces. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polymers have been shown to be interesting materials for the development of several devices [1–7], mainly due to the flexibility of tailoring their properties to match specific goals. Numerous methods have been explored to structure the surface of polymers, aiming at technological applications. Among them, laser micromachining has been receiving a great deal of attention due to its versatility and precision for structuring the surface as well as the bulk of materials [8–12]. For laser micromachining, in general, laser light is focused into the sample while it is translated or the laserbeam scanned. The light intensity achieved at the focus can lead to material’s changes or ablation. These changes can be beneficially employed to fabricate devices; for example, modification of the refractive index enables producing waveguides for integrating photonic devices [2,13,14]. The development of superhydrophobic surfaces has attracted considerable attention because of their potential applications in self-cleaning surfaces, water proof devices, low friction coatings and microfluidics [15–17]. The wettability of a surface depends on its chemical nature as well as its topology. Therefore, in the last few years effort has been made on strategies to roughening the surface of different materials, aiming at the design of superhydrophobic surfaces [18–26]; hydrophobic surfaces exhibit contact angle with water larger than 90◦ , while for superhydrophobic ones it should be higher than 150◦ [27]. Remarkable results have been achieved by Baldacchini et al. [21], where femtosecond pulses were used to produce microstructured superhydrophobic silicon surfaces. The
∗ Corresponding author. Tel.: +55 16 3373 8085. E-mail address: [email protected] (C.R. Mendonc¸a). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.10.156
same concept was employed in Ref. [28] to fabricate a silicon-based water repellent surface, which mimics the structure of the Lotus leaf. In this paper, we investigate the use of picoseconds laser pulses to microstructure the surface of polymeric films, in order to fabricate samples with controllable hydrophobicity. Using 70 ps pulses at 532 nm from a Q-switch/mode-locked Nd:YAG laser, we produced periodic surface microstructures (square-shaped pillars) with different features, and examined the dependence of the morphology on the static contact angle for water, which follows the Cassie–Baxter model. We were able to obtain a water contact angle of 157◦ , which means that the polymer surface turned superhydrophobic. 2. Experimental The commercial poly(1-methoxy-4-(O-disperse Red 1)-2,5bis(2-methoxyethyl) benzene) (PODR1) was dissolved in chloroform in a concentration of 2 wt.%. Samples with thicknesses of approximately 2 m were produced by casting the polymer solution on a glass substrate. PODR1, whose chemical structure is shown in Fig. 1, is a good candidate for laser microstructuring because of its good film forming properties and strong absorption in the visible region [15]. The UV–vis absorption spectrum of the sample was measured using a Varian Cary 17 spectrophotometer. PODR1 films were micromachined using 70-ps, 532-nm pulses from a Q-switched and mode-locked Nd:YAG laser, operating at a repetition rate of 850 Hz. Single pulses were extracted from the Q-switch pulse train employing a Pockels-cell-based pulse picker. Pulses with energies ranging from 17 nJ up to 65 nJ were focused, through 0.65-NA microscope objective, onto the sample surface, which was translated at a constant speed of 1 mm/s with respect
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Fig. 1. UV–vis absorption spectrum of a cast film of PODR1. The inset shows the chemical structure of PODR1.
to the laser beam. The micromachined samples were analyzed by optical microscopy and atomic force microscopy. To increase the sample’s natural hydrophobicity, following the micromachining, samples were treated with (heptadecafluoro1,1,2,2-tetrahydrodecyl)trichlorosilane for 24 h in a low-pressure chamber [21,23]. Such treatment allows one to study changes in hydrophobicity only due to the surface pattern produced. The same treatment was carried out with the not microstructured samples (flat samples). The surface’s wettability was determined by measuring the static contact angle of a water droplet using a goniometer coupled to a horizontal microscope. We used 3 L droplets for all experiments. The measurements were realized at room temperature (22 ◦ C) with the relative air humidity varying from 40 to 50%. 3. Results and discussions
Fig. 2. Optical microscopy of PODR1films on glass substrate microstructured with various periods: 10 m (a), 15 m (b) and 150 m (c).
The UV–vis absorption spectrum of a PODR1 film is displayed in Fig. 1. The band located at 485 nm corresponds to the → * electronic transition of the azochromophore. As can be seen, the sample presents high absorbance at 532 nm, the wavelength employed for the micromachining. We determined the threshold energy at which damage occurs as a result of focusing the Nd:YAG laser beam into the sample. By using a CCD camera we monitor the sample transmission in real time as the sample was micromachined. The transmission of visible light through the micromachined lines increases with pulse energy. The threshold energy for inducing visible modification in the PODR1 sample was measured to be 0.8 nJ, for a translation speed of 1 mm/s. For pulse energy of 3.5 nJ, micromachining results in ablation of the film, which was verified by the high contrast observed in the light transmission. Using laser micromachining we were able to fabricate polymeric surfaces presenting square-shaped-pillars morphologies with distinct periodicities, from = 5.0 m to = 500 m. In Fig. 2 we show optical microscopy images of microstructured PODR1 films surfaces with periodicities of 10 m (a), 15 m (b) and 150 m (c). The depth of micromachined grooves produced by ablation was determined by atomic force microscopy of the samples. In Fig. 3(a) we show a representative atomic force microscopy image of a PODR1 film microstructured with a period of 5 m. As can be seen in the cross-section analysis along a line of the microstructured region (sample depth profile), the depth of the pattern is of approximately 2 m. For pulse energies from 17 nJ to 65 nJ and speed of 1 mm/s, which correspond to the range of parameters used in this
work, groove depth are always of about 2 m, which is comparable to the sample thickness. From the atomic force micrographs we determined the surface roughness (arithmetic average – Ra). We observed an increase in the surface roughness from Ra = 100 nm, in the not microstructured surface, to Ra = 450 nm in the microstructured sample. The morphologies of the structured samples are basically defined by the spacing between two grooves, as well as the groove width. An important feature to observe in Fig. 2 is that we were able to generate morphologies with different ratios between the pillar cross-sectional area and the projected area of the microstructure, which can be interesting for wettability studies. Once the surface microstructuring was finished, fluorosilane was used to lower the surface energy, which allows investigating changes in hydrophobicity only related to the surface morphology produced. To evaluate the wetting properties of the microstructured samples, we measured the contact angle of 3 mm × 3 mm micromachined areas of the sample. Fig. 4 (a) and (b) shows, respectively, images of a water droplet on flat and microstructure surfaces ( = 20 m), both treated with fluorosilane. The contact angle of the flat surface is 108◦ , while it is (157 ± 3◦ ) on the microstructured surface, which corresponds to an increase of 49◦ . To compare the contact angle obtained for samples with different surface morphologies, in Fig. 5 we plot the contact angle (black circles) as a function of the parameter f, which is defined as f = SS /SP , where SS is the water/surface contact area and SP the total projected area both in a unitary cell of the periodic structure. The contact angle at f = 1 (108 ± 3◦ ) corresponds to the value for the
M.R. Cardoso et al. / Applied Surface Science 257 (2011) 3281–3284
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contact angle (degree)
180
160
140
120
100 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
f Fig. 5. Static contact angle of water (䊉) on microstructured surfaces as a function of the parameter f. The gray line represents the values obtained using the Cassie–Baxter model.
Two models have been used to describe the effect of roughening on the surface hydrophobicity: Cassie–Baxter and Wenzel models. According to the Wenzel model, the liquid is in contact with every part of the microstructured surface. In this case, the increase in the contact angle is due to the increase in the contact area relative to the flat surface. In the Wenzel model, the contact angle of the roughened surface, R , is given by Fig. 3. (a) Atomic force micrograph of the microstructured PODR1 film surface. (b) Cross-section analysis of the microstructured region, obtained along the red dotted line in (a).
unstructured surface (flat). Such value indicates that water does not wet this surface, since the contact angle is greater than 90◦ . As seen in Fig. 5, the contact angle of the microstructured surface increases by 49◦ at f = 0.135, which corresponds to a structuring periodicity of 20 m. As f increases the contact angle decreases, approaching the value of the flat surface as f approaches 1. We observed a small contact angle hysteresis, meaning that there is negligible difference between the advancing and receding contact angle, an important feature in the engineering of hydrophobic surfaces. For the surface microstructured with a period of 5 m, for example, we observed an advancing contact angle of 157◦ and a receding contact angle of 153◦ , which correspond to a hysteresis of 4◦ .
cos R = r cos
(1)
where r is the ratio between the actual and projected contact surface and is the contact angle measured on the flat surface. Since r is greater than 1, this model predicts an increase on the contact angle for non-wetting surfaces ( > 90◦ ) upon surface roughening. By applying the Wenzel model to the microstructured surfaces produced, considering the higher value for r (r = 1.88) we obtain R = 130◦ , that is much smaller than the ones we have experimentally observed. This indicates that the Wenzel model is not able to describe our experimental data. In the Cassie–Baxter model [29,30], it is assumed that the liquid does not entirely wet a roughened surface because air is trapped in the surface gaps. In such case, there is a three-phase contact (solid–liquid–gas), where the liquid interacts with the polymer and with air. For to this model, large contact angles are observed when the contact area between the solid and the liquid is diminished. In the Cassie–Baxter model, the contact angle of the roughened
Fig. 4. Pictures of water droplets on flat (a) and microstructured ( = 20 m) (b) surfaces treated with fluorosilane.
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surface, R , is given by cos R = f (cos + 1) − 1
(2)
where f, as defined earlier, is the fraction of the solid in contact with the liquid and is the contact angle determined on the flat surface. In this case, the smaller the value of f the larger is the increase in the measured contact angle. The gray line with circles in Fig. 5 represents the values obtained using the Cassie–Baxter model with the values of f that describe the surface morphologies we produced by laser microstructuring and using = 108◦ . As can be seen the experimental data are consistent with the Cassie–Baxter model. The small contact area between the water droplet and the surface of the pillars result in a small value for f, which gives rise to a large contact angle, in accordance with Eq. (2). In this situation, the water droplet does not wet the surface, being able to roll on it. 4. Conclusion We were able to fabricate surfaces with distinct wetting properties by microstructuring a polymeric material with picoseconds laser pulses. We demonstrate the fabrication of superhydrophobic surfaces with contact angle higher than 150◦ . The dependence of the contact angle on the surface morphology is in agreement with the Cassie–Baxter model, indicating that the aspect ratio of the structured surface is such that the entrapped air pockets lead to a small contact area between water and polymer and, consequently, to a larger increase in the contact angle. Therefore, the surface structuring method presented here seems to be an interesting option to control the wetting properties of polymeric surfaces. The morphology of the microstructured areas can be controlled, allowing the design of polymeric surfaces with distinct contact angles for water, even reaching the superhydrophobic limit. Acknowledgements Financial support from FAPESP (Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo), CNPQ (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and the Air Force Office of Scientific Research (FA9550-07-1-0374) are acknowledged. Technical assistance from André L.S. Romero and Dr. Ivana A. Borin (FFCLRP – USP) are gratefully acknowledged. References [1] Y. Li, K. Yamada, T. Ishizuka, W. Watanabe, K. Itoh, Z.X. Zhou, Single femtosecond pulse holography using polymethyl methacrylate, Opt. Express 10 (2002) 1173–1178. [2] C.R. Mendonca, S. Orlando, G. Cosendey, M. Winkler, E. Mazur, Femtosecond laser micromachining in the conjugated polymer MEH-PPV, Appl. Surf. Sci. 254 (2007) 1135–1139. [3] S. Sowa, W. Watanabe, T. Tamaki, J. Nishii, K. Itoh, Symmetric waveguides in poly(methyl methacrylate) fabricated by femtosecond laser pulses, Opt. Express 14 (2006) 291–297. [4] D.B. Wolfe, J.B. Ashcom, J.C. Hwang, C.B. Schaffer, E. Mazur, G.M. Whitesides, Customization of poly(dimethylsiloxane) stamps by micromachining using a femtosecond-pulsed laser, Adv. Mater. 15 (2003) 62–65.
[5] A. Zoubir, C. Lopez, M. Richardson, K. Richardson, Femtosecond laser fabrication of tubular waveguides in poly(methyl methacrylate), Opt. Lett. 29 (2004) 1840–1842. [6] P. Tayalia, C.R. Mendonca, T. Baldacchini, D.J. Mooney, E. Mazur, 3D cellmigration studies using two-photon engineered polymer scaffolds, Adv. Mater. 20 (2008) 4494–4498. [7] C.J. Hayden, C. Dalton, Direct patterning of microelectrode arrays using femtosecond laser micromachining, Appl. Surf. Sci. 256 (2010) 3761–3766. [8] D. Cristea, P. Obreja, M. Kusko, E. Manea, R. Rebigan, Polymer micromachining for micro- and nanophotonics, Mater. Sci. Eng. C: Biomimetic Supramol. Syst. 26 (2006) 1049–1055. [9] N.G. Semaltianos, W. Perrie, P. French, M. Sharp, G. Dearden, K.G. Watkins, Femtosecond laser surface texturing of a nickel-based superalloy, Appl. Surf. Sci. 255 (2008) 2796–2802. [10] H. Nishiyama, J. Nishii, M. Mizoshiri, Y. Hirata, Microlens arrays of highrefractive-index glass fabricated by femtosecond laser lithography, Appl. Surf. Sci. 255 (2009) 9750–9753. [11] R.R. Gattass, E. Mazur, Femtosecond laser micromachining in transparent materials, Nat. Photon. 2 (2008) 219–225. [12] A. Ovsianikov, A. Ostendorf, B.N. Chichkov, Three-dimensional photofabrication with femtosecond lasers for applications in photonics and biomedicine, Appl. Surf. Sci. 253 (2007) 6599–6602. [13] L.M. Tong, R.R. Gattass, I. Maxwell, J.B. Ashcom, E. Mazur, Optical loss measurements in femtosecond laser written waveguides in glass, Opt. Commun. 259 (2006) 626–630. [14] C.R. Mendonca, L.R. Cerami, T. Shih, R.W. Tilghman, T. Baldacchini, E. Mazur, Femtosecond laser waveguide micromachining of PMMA films with azoaromatic chromophores, Opt. Express 16 (2008) 200–206. [15] M.R. Cardoso, V. Tribuzi, D.T. Balogh, L. Misoguti, C.R. Mendonca, Laser microstructuring of azopolymers via surface relief gratings: controlling hydrophobicity, J. Optoelectron. Adv. Mater. 12 (2010) 745–748. [16] Z.Q. Yuan, H. Chen, J. Zhang, Facile method to prepare lotus-leaf-like superhydrophobic poly(vinyl chloride) film, Appl. Surf. Sci. 254 (2008) 1593– 1598. [17] S. Kwon, J.W. Ha, J. Noh, S.Y. Lee, Patterning of polypyrrole using a fluoropolymer as an adsorption-protecting molecule, Appl. Surf. Sci. 257 (2010) 165–171. [18] R. Blossey, Self-cleaning surfaces – virtual realities, Nat. Mater. 2 (2003) 301–306. [19] T.L. Sun, L. Feng, X.F. Gao, L. Jiang, Bioinspired surfaces with special wettability, Accounts Chem. Res. 38 (2005) 644–652. [20] R. Furstner, W. Barthlott, C. Neinhuis, P. Walzel, Wetting and self-cleaning properties of artificial superhydrophobic surfaces, Langmuir 21 (2005) 956– 961. [21] T. Baldacchini, J.E. Carey, M. Zhou, E. Mazur, Superhydrophobic surfaces prepared by microstructuring of silicon using a femtosecond laser, Langmuir 22 (2006) 4917–4919. [22] D. Oner, T.J. McCarthy, Ultrahydrophobic surfaces. Effects of topography length scales on wettability, Langmuir 16 (2000) 7777–7782. [23] W.L. Wu, Q.Z. Zhu, F.L. Qing, C.C. Han, Water repellency on a fluorine-containing polyurethane surface: toward understanding the surface self-cleaning effect, Langmuir 25 (2009) 17–20. [24] M. Barberoglou, V. Zorba, E. Stratakis, E. Spanakis, P. Tzanetakis, S.H. Anastasiadis, C. Fotakis, Bio-inspired water repellent surfaces produced by ultrafast laser structuring of silicon, Appl. Surf. Sci. 255 (2009) 5425–5429. [25] Y.L. Yang, C.C. Hsu, T.L. Chang, L.S. Kuo, P.H. Chen, Study on wetting properties of periodical nanopatterns by a combinative technique of photolithography and laser interference lithography, Appl. Surf. Sci. 256 (2010) 3683– 3687. [26] B. Wu, M. Zhou, J. Li, X. Ye, G. Li, L. Cai, Superhydrophobic surfaces fabricated by microstructuring of stainless steel using a femtosecond laser, Appl. Surf. Sci. 256 (2009) 61–66. [27] W. Li, A. Amirfazli, Microtextured superhydrophobic surfaces: a thennodynarnic analysis, Adv. Colloid Interface Sci. 132 (2007) 51–68. [28] V. Zorba, E. Stratakis, M. Barberoglou, E. Spanakis, P. Tzanetakis, S.H. Anastasiadis, C. Fotakis, Biomimetic artificial surfaces quantitatively reproduce the water repellency of a lotus leaf, Adv. Mater. 20 (2008) 4049– 4054. [29] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 0546–0550. [30] N.A. Patankar, On the modeling of hydrophobic contact angles on rough surfaces, Langmuir 19 (2003) 1249–1253.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laser microstructuring for fabricating superhydrophobic polymeric surfaces M.R. Cardoso, V. Tribuzi, D.T. Balogh, L. Misoguti, C.R. Mendonc¸a ∗ Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, CEP 13560 970 São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Received 23 August 2010 Received in revised form 30 October 2010 Accepted 31 October 2010 Available online 5 November 2010 Keywords: Laser micromachining Polymers Superhydrophobic surfaces
a b s t r a c t In this paper we show the fabrication of hydrophobic polymeric surfaces through laser microstructuring. By using 70-ps pulses from a Q-switched and mode-locked Nd:YAG laser at 532 nm, we were able to produce grooves with different width and separation, resulting in square-shaped pillar patterns. We investigate the dependence of the morphology on the surface static contact angle for water, showing that it is in agreement with the Cassie–Baxter model. We demonstrate the fabrication of a superhydrophobic polymeric surface, presenting a water contact angle of 157◦ . The surface structuring method presented here seems to be an interesting option to control the wetting properties of polymeric surfaces. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polymers have been shown to be interesting materials for the development of several devices [1–7], mainly due to the flexibility of tailoring their properties to match specific goals. Numerous methods have been explored to structure the surface of polymers, aiming at technological applications. Among them, laser micromachining has been receiving a great deal of attention due to its versatility and precision for structuring the surface as well as the bulk of materials [8–12]. For laser micromachining, in general, laser light is focused into the sample while it is translated or the laserbeam scanned. The light intensity achieved at the focus can lead to material’s changes or ablation. These changes can be beneficially employed to fabricate devices; for example, modification of the refractive index enables producing waveguides for integrating photonic devices [2,13,14]. The development of superhydrophobic surfaces has attracted considerable attention because of their potential applications in self-cleaning surfaces, water proof devices, low friction coatings and microfluidics [15–17]. The wettability of a surface depends on its chemical nature as well as its topology. Therefore, in the last few years effort has been made on strategies to roughening the surface of different materials, aiming at the design of superhydrophobic surfaces [18–26]; hydrophobic surfaces exhibit contact angle with water larger than 90◦ , while for superhydrophobic ones it should be higher than 150◦ [27]. Remarkable results have been achieved by Baldacchini et al. [21], where femtosecond pulses were used to produce microstructured superhydrophobic silicon surfaces. The
∗ Corresponding author. Tel.: +55 16 3373 8085. E-mail address: [email protected] (C.R. Mendonc¸a). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.10.156
same concept was employed in Ref. [28] to fabricate a silicon-based water repellent surface, which mimics the structure of the Lotus leaf. In this paper, we investigate the use of picoseconds laser pulses to microstructure the surface of polymeric films, in order to fabricate samples with controllable hydrophobicity. Using 70 ps pulses at 532 nm from a Q-switch/mode-locked Nd:YAG laser, we produced periodic surface microstructures (square-shaped pillars) with different features, and examined the dependence of the morphology on the static contact angle for water, which follows the Cassie–Baxter model. We were able to obtain a water contact angle of 157◦ , which means that the polymer surface turned superhydrophobic. 2. Experimental The commercial poly(1-methoxy-4-(O-disperse Red 1)-2,5bis(2-methoxyethyl) benzene) (PODR1) was dissolved in chloroform in a concentration of 2 wt.%. Samples with thicknesses of approximately 2 m were produced by casting the polymer solution on a glass substrate. PODR1, whose chemical structure is shown in Fig. 1, is a good candidate for laser microstructuring because of its good film forming properties and strong absorption in the visible region [15]. The UV–vis absorption spectrum of the sample was measured using a Varian Cary 17 spectrophotometer. PODR1 films were micromachined using 70-ps, 532-nm pulses from a Q-switched and mode-locked Nd:YAG laser, operating at a repetition rate of 850 Hz. Single pulses were extracted from the Q-switch pulse train employing a Pockels-cell-based pulse picker. Pulses with energies ranging from 17 nJ up to 65 nJ were focused, through 0.65-NA microscope objective, onto the sample surface, which was translated at a constant speed of 1 mm/s with respect
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Fig. 1. UV–vis absorption spectrum of a cast film of PODR1. The inset shows the chemical structure of PODR1.
to the laser beam. The micromachined samples were analyzed by optical microscopy and atomic force microscopy. To increase the sample’s natural hydrophobicity, following the micromachining, samples were treated with (heptadecafluoro1,1,2,2-tetrahydrodecyl)trichlorosilane for 24 h in a low-pressure chamber [21,23]. Such treatment allows one to study changes in hydrophobicity only due to the surface pattern produced. The same treatment was carried out with the not microstructured samples (flat samples). The surface’s wettability was determined by measuring the static contact angle of a water droplet using a goniometer coupled to a horizontal microscope. We used 3 L droplets for all experiments. The measurements were realized at room temperature (22 ◦ C) with the relative air humidity varying from 40 to 50%. 3. Results and discussions
Fig. 2. Optical microscopy of PODR1films on glass substrate microstructured with various periods: 10 m (a), 15 m (b) and 150 m (c).
The UV–vis absorption spectrum of a PODR1 film is displayed in Fig. 1. The band located at 485 nm corresponds to the → * electronic transition of the azochromophore. As can be seen, the sample presents high absorbance at 532 nm, the wavelength employed for the micromachining. We determined the threshold energy at which damage occurs as a result of focusing the Nd:YAG laser beam into the sample. By using a CCD camera we monitor the sample transmission in real time as the sample was micromachined. The transmission of visible light through the micromachined lines increases with pulse energy. The threshold energy for inducing visible modification in the PODR1 sample was measured to be 0.8 nJ, for a translation speed of 1 mm/s. For pulse energy of 3.5 nJ, micromachining results in ablation of the film, which was verified by the high contrast observed in the light transmission. Using laser micromachining we were able to fabricate polymeric surfaces presenting square-shaped-pillars morphologies with distinct periodicities, from = 5.0 m to = 500 m. In Fig. 2 we show optical microscopy images of microstructured PODR1 films surfaces with periodicities of 10 m (a), 15 m (b) and 150 m (c). The depth of micromachined grooves produced by ablation was determined by atomic force microscopy of the samples. In Fig. 3(a) we show a representative atomic force microscopy image of a PODR1 film microstructured with a period of 5 m. As can be seen in the cross-section analysis along a line of the microstructured region (sample depth profile), the depth of the pattern is of approximately 2 m. For pulse energies from 17 nJ to 65 nJ and speed of 1 mm/s, which correspond to the range of parameters used in this
work, groove depth are always of about 2 m, which is comparable to the sample thickness. From the atomic force micrographs we determined the surface roughness (arithmetic average – Ra). We observed an increase in the surface roughness from Ra = 100 nm, in the not microstructured surface, to Ra = 450 nm in the microstructured sample. The morphologies of the structured samples are basically defined by the spacing between two grooves, as well as the groove width. An important feature to observe in Fig. 2 is that we were able to generate morphologies with different ratios between the pillar cross-sectional area and the projected area of the microstructure, which can be interesting for wettability studies. Once the surface microstructuring was finished, fluorosilane was used to lower the surface energy, which allows investigating changes in hydrophobicity only related to the surface morphology produced. To evaluate the wetting properties of the microstructured samples, we measured the contact angle of 3 mm × 3 mm micromachined areas of the sample. Fig. 4 (a) and (b) shows, respectively, images of a water droplet on flat and microstructure surfaces ( = 20 m), both treated with fluorosilane. The contact angle of the flat surface is 108◦ , while it is (157 ± 3◦ ) on the microstructured surface, which corresponds to an increase of 49◦ . To compare the contact angle obtained for samples with different surface morphologies, in Fig. 5 we plot the contact angle (black circles) as a function of the parameter f, which is defined as f = SS /SP , where SS is the water/surface contact area and SP the total projected area both in a unitary cell of the periodic structure. The contact angle at f = 1 (108 ± 3◦ ) corresponds to the value for the
M.R. Cardoso et al. / Applied Surface Science 257 (2011) 3281–3284
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contact angle (degree)
180
160
140
120
100 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
f Fig. 5. Static contact angle of water (䊉) on microstructured surfaces as a function of the parameter f. The gray line represents the values obtained using the Cassie–Baxter model.
Two models have been used to describe the effect of roughening on the surface hydrophobicity: Cassie–Baxter and Wenzel models. According to the Wenzel model, the liquid is in contact with every part of the microstructured surface. In this case, the increase in the contact angle is due to the increase in the contact area relative to the flat surface. In the Wenzel model, the contact angle of the roughened surface, R , is given by Fig. 3. (a) Atomic force micrograph of the microstructured PODR1 film surface. (b) Cross-section analysis of the microstructured region, obtained along the red dotted line in (a).
unstructured surface (flat). Such value indicates that water does not wet this surface, since the contact angle is greater than 90◦ . As seen in Fig. 5, the contact angle of the microstructured surface increases by 49◦ at f = 0.135, which corresponds to a structuring periodicity of 20 m. As f increases the contact angle decreases, approaching the value of the flat surface as f approaches 1. We observed a small contact angle hysteresis, meaning that there is negligible difference between the advancing and receding contact angle, an important feature in the engineering of hydrophobic surfaces. For the surface microstructured with a period of 5 m, for example, we observed an advancing contact angle of 157◦ and a receding contact angle of 153◦ , which correspond to a hysteresis of 4◦ .
cos R = r cos
(1)
where r is the ratio between the actual and projected contact surface and is the contact angle measured on the flat surface. Since r is greater than 1, this model predicts an increase on the contact angle for non-wetting surfaces ( > 90◦ ) upon surface roughening. By applying the Wenzel model to the microstructured surfaces produced, considering the higher value for r (r = 1.88) we obtain R = 130◦ , that is much smaller than the ones we have experimentally observed. This indicates that the Wenzel model is not able to describe our experimental data. In the Cassie–Baxter model [29,30], it is assumed that the liquid does not entirely wet a roughened surface because air is trapped in the surface gaps. In such case, there is a three-phase contact (solid–liquid–gas), where the liquid interacts with the polymer and with air. For to this model, large contact angles are observed when the contact area between the solid and the liquid is diminished. In the Cassie–Baxter model, the contact angle of the roughened
Fig. 4. Pictures of water droplets on flat (a) and microstructured ( = 20 m) (b) surfaces treated with fluorosilane.
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surface, R , is given by cos R = f (cos + 1) − 1
(2)
where f, as defined earlier, is the fraction of the solid in contact with the liquid and is the contact angle determined on the flat surface. In this case, the smaller the value of f the larger is the increase in the measured contact angle. The gray line with circles in Fig. 5 represents the values obtained using the Cassie–Baxter model with the values of f that describe the surface morphologies we produced by laser microstructuring and using = 108◦ . As can be seen the experimental data are consistent with the Cassie–Baxter model. The small contact area between the water droplet and the surface of the pillars result in a small value for f, which gives rise to a large contact angle, in accordance with Eq. (2). In this situation, the water droplet does not wet the surface, being able to roll on it. 4. Conclusion We were able to fabricate surfaces with distinct wetting properties by microstructuring a polymeric material with picoseconds laser pulses. We demonstrate the fabrication of superhydrophobic surfaces with contact angle higher than 150◦ . The dependence of the contact angle on the surface morphology is in agreement with the Cassie–Baxter model, indicating that the aspect ratio of the structured surface is such that the entrapped air pockets lead to a small contact area between water and polymer and, consequently, to a larger increase in the contact angle. Therefore, the surface structuring method presented here seems to be an interesting option to control the wetting properties of polymeric surfaces. The morphology of the microstructured areas can be controlled, allowing the design of polymeric surfaces with distinct contact angles for water, even reaching the superhydrophobic limit. Acknowledgements Financial support from FAPESP (Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo), CNPQ (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and the Air Force Office of Scientific Research (FA9550-07-1-0374) are acknowledged. Technical assistance from André L.S. Romero and Dr. Ivana A. Borin (FFCLRP – USP) are gratefully acknowledged. References [1] Y. Li, K. Yamada, T. Ishizuka, W. Watanabe, K. Itoh, Z.X. Zhou, Single femtosecond pulse holography using polymethyl methacrylate, Opt. Express 10 (2002) 1173–1178. [2] C.R. Mendonca, S. Orlando, G. Cosendey, M. Winkler, E. Mazur, Femtosecond laser micromachining in the conjugated polymer MEH-PPV, Appl. Surf. Sci. 254 (2007) 1135–1139. [3] S. Sowa, W. Watanabe, T. Tamaki, J. Nishii, K. Itoh, Symmetric waveguides in poly(methyl methacrylate) fabricated by femtosecond laser pulses, Opt. Express 14 (2006) 291–297. [4] D.B. Wolfe, J.B. Ashcom, J.C. Hwang, C.B. Schaffer, E. Mazur, G.M. Whitesides, Customization of poly(dimethylsiloxane) stamps by micromachining using a femtosecond-pulsed laser, Adv. Mater. 15 (2003) 62–65.
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